Metal hardening technique for custom blanks

Manufacturers use tailored blanks – sheet-metal pieces with varying thickness – to create complex parts in stamping operations. Producers tend to use laser-welding systems to join different gauges to create the thickness differences (laser-welded blanks) or rolling equipment to plastically deform the metal into different gauges (tailored-rolled blanks).

Dr. Andrzej Rosochowski, a reader in design, manufacture, and engineering Management at the University of Strathclyde in Glasgow, Scotland, has a new technology to add to that mix – tailored-sheared blanks. In this process, sheet metal gets fed into a reciprocating press and forced through an angled die, putting shear strain on the metal. By adjusting the stroke of the press, Rosochowski’s process can alter the gauges of the metal being sheared.

“You can tailor the amount of strengthening by changing the angle in the die, that is changing the amount of plastic strain. You can also thin the material or make it thicker than the original sheet. You can even create thickness steps on both sides of the blank instead of one side being flat,” Rosochowski says.

ECAP variation

Rosochowski has spent the past 15 years studying how to practically shorten the grain sizes of metals. Smaller-grain metals are stronger than traditional ones, yet still easily formable into complex shapes. With some hard-to-process metals such as magnesium, shortening grain sizes can increase strength and improve ductility. He calls grain tailoring an alternative to heat treatment and alloying – processes that improve strength but can harm formability.

One method to shorten grain size is equal channel angular pressing (ECAP), forcing metal sheets through an L-shaped die to produce shear strain in the metal – to shrink grain sizes to less than 1µm.

Rosochowski explains. “When it comes from the die, it has basically the same cross-section as the initial billet because the channels are equal. Both arms of this letter L have the same dimensions.”

His variation on ECAP involves splitting the die into two pieces, one stationary, the other reciprocating. The movement of the reciprocating die is synchronized by incrementaly feeding the material in the stationary die. By changing the bottom position of the reciprocating die, the material leaving the pressing can have a different thickness than the one going in. By varying that position during the process, the outgoing sheet can go through multiple stair-step like thickness changes.

For blank producers, the process could solve some problems with the rolling and welding processes. In welding, the weld seam between different-gauge sheets can have impurities or weak points. And in rolling, the stair steps between thickness levels tend to be fairly gradual. The sheering process can have steep transitions and includes no welding artifacts.

Custom mechanical properties

The biggest opportunity, Rosochowski says, is the ability to control mechanical properties throughout a piece. Automakers use tailored blanks to make parts that have high-strength, critical areas surrounded by lower-strength support material. By controlling grain size via the sheering process, sheered blanks could have thick, strong areas that have been run through multiple grain-shortening steps, surrounded by thinner gauges of the same metal that haven’t been forced through the die.

“You could even have a uniform-thickness sheet of metal with regions that have different properties,” Rosochowski says. “The metals could be aluminum, steel, titanium, and possibly magnesium. Magnesium is about 30% lighter than aluminum, and it has been tried a couple of times for automotive applications, but the main obstacle is the low ductility of magnesium alloys. Ductility improves with the shearing process, so it might become more viable.”

Adaptation, next steps

Rosochowski says his process isn’t exactly a new concept. For thousands of years, blacksmiths have pounded on metal and folded it over on itself to increase hardness – a grain-shortening process. By splitting the L-shaped die, his process even looks much like a blacksmith at his forge – the material feeds through one side and gets hammered into shape by the opposing side of the die.

One challenge in developing the process is its productivity, as the material advances in the die, the reciprocating tool moves away. When the stamping takes place, feeding must stop so clamping can be engaged. The number of steps needed to produce a sheet depends on the feeding stroke, which is limited to 30% of sheet thickness. With faster and better controlled press actuators, he envisions a more synchronized motion in which feeding could take place faster. High-frequency pressing tools and other systems could help, he adds.

“I’m looking for partners to develop this further,” Rosochowski says. “We need to study how to improve speeds and how different materials respond to this.”

University of Strathclyde Glasgow

www.strath.ac.uk